Daewoo DUB-2850GB Service Manual page 45

CP-780 Service Manual
5.2.8 Chroma, luminance and feature processing
Some versions contain a 4H/2H(2D) adaptive PAL/NTSC comb filter. The comb filter is
automatically activated when standard CVBS signals are received. A signal is considered as
standard signal when a PAL or NTSC signal is identified and when the vertical divider is in
the modes standard narrow window or standard TV norm . For non-standard signals and for
SECAM signals the comb filter is bypassed and the signal is filtered by means of bandpass and
trap filters.
The chroma band-pass and trap circuits (including the SECAM cloche filter) are realised by
means of internal filters and are tuned to the right frequency by comparing the tuning frequency
with the reference frequency of the colour decoder.
The circuit contains the following picture improvement features:
* Peaking control circuit. The peaking function can be activated for all incoming CVBS, Y/C and
RGB/YPrPb signals. Various parameters of the peaking circuit can be adapted by means of the
I2C-bus. The main parameters are:
- Peaking centre frequency (via the PF1/PF0 bits in subaddress 19H).
- Ratio of positive and negative peaks (via the RPO1/RPO0 bits in subaddress 47H). The peaks
in the direction white are the positive peaks.
- Ratio of pre- and after shoots (via the RPA1/RPA0 bits in subaddress 47H).
* Video dependent coring in the peaking circuit. The coring can be activated only in the low-light
parts of the screen. This effectively reduces noise while having maximum peaking in the bright
parts of the picture.
* Black stretch. This function corrects the black level for incoming signals which have a
difference between the black level and the blanking level. The amount of stretching (A-A in Fig.
72) and the minimum required back ground to activate the stretching can be set by means of
the I2C-bus (BSD/AAS in subaddress 45H).
* Gamma control. When this function is active the transfer characteristic of the luminance
amplifier is made non-linear. The control curve can be adapted by means of I2C-bus settings
(see Fig. 74). It is possible to make the gamma control function dependent on the picture
content (Average Picture Level, APL). The effect is illustrated in Fig. 75. Previously this function
was mentioned under the name white stretch function .
* Blue-stretch. This circuit is intended to shift colour near white with sufficient contrast values
towards more blue to obtain a brighter impression of the picture.
* Dynamic skin tone (flesh) control. This function is realised in the YUV domain by detecting the
colours near to the skin tone.
* Scan-Velocity modulation output. Also the SVM function can be activated for all incoming
CVBS, Y/C and RGB/YPrPb signals. The delay between the RGB output signals and the SVM
output signal can be adjusted (by means of the SVM2-SVM0 bits in subaddress 48H) so that an
optimum picture performance can be obtained. Furthermore a coring function can be activated.
It is possible to generate Scan Velocity Modulation drive signals during the display of full
screen teletext (not in mixed mode). Another feature is that the SVM output signal can be
made dependent on the horizontal position on the screen (parabola on the SVM output).
-45-